Frontiers and Controversies in Astrophysics: Lecture 22 Transcript
April 19, 2007
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Professor Charles Bailyn:
We had gotten, at the end of last time, to the Big Rip. This is where the
ever-expanding Cosmological Constant--which is, of course, a contradiction in terms. If you imagine that
dark energy is not a Cosmological Constant, that it's actually increasing in time, rips everything to shreds.
The Universe becomes infinite in size in a finite amount of time and everything gets pulled apart. What I
want to start by doing today is to go back and do that again slowly, okay? Because I think this is not a simple
line of reasoning that leads you to this.
So, let's go all the way back to what we know about the Universe from observing galaxies, supernovas, things
like that. We know that the Universe is expanding and that, we know. Hubble already figured that out. We
know this from the Hubble Law, and from other--just, in general, from the study of standard candles at
relatively low redshift. So, distances out to, I don't know,
Z
of less than 0.2, or so. You can't tell the difference
between an accelerating or a decelerating Universe. All you know is that it's expanding, and you can figure
out how fast it's expanding.
Now, the next question then becomes the rate of expansion. Is the rate of expansion changing? Is it
accelerating? Is it decelerating? What's going on there? And so, you want to compare acceleration versus
deceleration, which is a fancy word, as you know, for slowing down. And deceleration: this is relatively easy
to grasp. This is what happens because of matter. Matter exists. It's got gravitational force, and gravitational
force tends to hold things together. So, if you've got something that's moving apart, and there's some
gravitational force, it'll tend to hold it together, and thus, slow it down.
Most of the matter, it turns out, is this weird dark matter, which we don't understand, but that's what's
responsible for the deceleration. If you're going to make it go faster, you need something much weirder that
pushes outward, that has repulsive force. This, we believe, actually exists. This, we label dark energy. And so,
the question of, "is it accelerating or is it decelerating," is basically a question of how much dark energy is
there versus how much matter is there, because the more--if you have more of this than this, then it will
accelerate and vice versa.
And what we have discovered by supernovae observations with redshifts of greater than 0.3, and by now, out
to about a redshift of 1, or so, demonstrate that--well, what exactly does this say? Let's go slowly. In the past,
the Universe was expanding more slowly than it is now, and therefore--those three dots are the mathematical
symbol for therefore. Therefore, the Universe is accelerating, and therefore, there's more dark energy than
there is matter.